Scalable coding

ABSTRACT

The invention proposes a method of and a device for coding a signal (S) to obtain a scalable bit-stream (O). The signal (S) comprises blocks of values. Each block is represented ( 20 ) as a sequence of bit planes and the values are scanned and transmitted ( 21, 23 ) in an order of decreasing bit plane significance. For each bit plane the scanning and transmitting ( 21,23 ) are performed in a rectangular scan zone (R MAX /C MAX ) starting from a corner of the block (usually an upper-left corner). Preferably, the scanning and transmitting ( 21,23 ) is performed on each block individually. The produced bit-stream (O) is quantized to a desired bit-rate by simple truncating ( 3 ) the bit-stream (O) at a desired position.

The invention relates to a method of and a device for scalable coding.

The invention further relates to an encoder, a camera system, a methodof decoding, a scalable decoder, a receiver, a scalable bit-stream and astorage medium.

WO 99/16250 discloses an embedded DCT-based still image codingalgorithm. An embedded bit-stream is produced by the encoder. Thedecoder can cut the bit-stream at any point and therefore reconstruct animage at a lower bit-rate. Since an embedded bit-stream contains alllower rates embedded at the beginning of the bit-stream, the bits areordered from the most important to the less important. Using an embeddedcode, the encoding simply stops when the target parameter as the bitcount is met. In a similar manner, given the embedded bit-stream, thedecoder can cease decoding at any point and can produce reconstructionscorresponding to all lower-rate encoding. The quality of thereconstructed image at this lower rate is the same as if the image wascoded directly at that rate.

The DCT is orthonormal, which means that it preserves the energy. Anerror in the transformed image of certain amplitude will produce anerror of the same magnitude in the original image. This means that thecoefficients with the largest magnitudes should be transmitted firstbecause they have the largest content of the information. This meansthat the information can also be ranked according to its binaryrepresentation, and the most significant bits should be transmittedfirst.

Coding is done bit-plane by bit-plane. The DCT coefficients are scannedand transmitted in an order starting from the upper left corner(corresponding to the DC coefficient) and ending in the lower rightcorner of each DCT block, i.e. from the lowest frequency coefficient tothe highest frequency coefficient. Inside a block, the DCT coefficientsare scanned in a diagonal order, bit plane by bit plane. After eachscanned diagonal, a flag is sent telling if there are any newsignificant coefficients in the rest of the block.

An object of the invention is, inter alia, to provide more efficientencoding. To this end, the invention provides a method of and a devicefor coding a signal, an encoder, a camera system, a method of decoding,a scalable decoder, a receiver, a scalable bit-stream and a storagemedium as defined in the independent claims. Advantageous embodimentsare defined in the dependent claims.

According to a first aspect of the invention, a signal comprising blocksof values is coded to obtain a scalable bit-stream by: representing eachblock as a sequence of bit planes, wherein most significant bits of thevalues form a most significant bit plane and respective less significantbits of the values form respective less significant bit planes; andscanning and transmitting the values in an order of decreasing bit planesignificance, wherein for each bit plane the step of scanning andtransmitting is performed in a rectangular scan zone starting from acorner of the block. The corner position depends on the way thecoefficients are ordered. Usually, the scan zone starts in an upper leftcorner of the block. The invention is based on the insight that data forindividual blocks may have a bias for either the horizontal or verticaldirection. This is especially the case if the values are transformcoefficients, but may also be true for other values. Therefore arectangular scan zone starting in a corner produces a more efficientencoding of the values of the blocks. For images, the inventionminimizes necessary image memory in video/image encoders, decoders andtransmission of image/video data along channels. The scanning inside thescan zone can be done in any fashion, as long as the encoder and thedecoder are synchronized on this scanning. The invention is especiallyapplicable in the field of low-cost, hardware video compression.

A second embodiment according to the invention is characterized in thatthe coding is performed on each block individually. Using therectangular scan zone is suitable for processing individual blocks. Anadvantage of processing individual blocks is that it offers thepossibility to work “on the fly” on each received block without the needfor gathering and rearranging all blocks of the signal. This reduces theamount of implementation memory. Because the blocks are codedindependently, they can be processed in parallel.

In a further embodiment of the invention the step of scanning andtransmitting comprises:

-   -   initially marking all values insignificant; and    -   performing the following steps for each bit plane until a stop        criterion is met:        -   transmitting a bit for each significant value in a current            bit-plane;        -   transmitting an indication whether or not any insignificant            values become newly significant in the current bit plane;            and        -   selecting and transmitting dimensions of the rectangular            scan zone for the newly significant values in the current            bit plane, followed by an indication for each not previously            significant value inside the scan zone whether the value has            become newly significant and a sign bit for each newly            significant coefficient.

The order of the steps in this embodiment may be changed withoutdeparting from the scope of the invention. Newly significant values aremarked such that for a next bit-plane they are regarded significant. Aslong as no value has become newly significant in a previous bit plane,no significant values exist. For such a bit-plane no bits aretransmitted for significant values. This is, e.g., the case for a mostsignificant bit-plane.

In an embodiment of the invention, the device is used in a hybridencoder, the hybrid encoder further comprising a truncator fortruncating the scalable bit-stream to obtain an output signal at acertain bit-rate.

In an advantageous embodiment, the device is used in an encoder tofurnish the scalable bit-stream to a memory for storing a previousframe. This minimizes the necessary memory, which makes it more feasibleto integrate a hybrid coder with the memory on a single chip. A hybridencoder is an encoder that codes both spatially and temporally by, e.g.,a two-dimensional data transformation and motion compensation.

The aforementioned and other aspects of the invention will be apparentfrom and elucidated with reference to the embodiments describedhereinafter.

In the drawings:

FIG. 1 shows an exemplary bit-plane with a rectangular scan zoneaccording to the invention;

FIG. 2 shows a visualization of a coding technique according to theinvention;

FIG. 3 shows an exemplary embodiment according to the invention;

FIG. 4 shows an example of a scalable image coding method according tothe invention;

FIGS. 5 and 6 show hybrid encoders according to the invention applied ina camera system, wherein the hybrid encoders use a scalable coder tofurnish a scalable bit-stream to a memory;

FIG. 7 shows a camera system comprising a further hybrid encoderaccording to the invention which uses a scalable coder to furnish ascalable bit-stream to an output of the hybrid encoder; and

FIG. 8 shows a decoder for decoding the scalable bit-stream produced bythe hybrid encoder of FIG. 7.

The drawings only show those elements that are necessary to understandthe invention.

In order to generate a bit-stream that can be truncated, the mostsignificant information should be transmitted first, followed bysubsequent refinement information. In case of Discrete CosineTransformation (DCT) coding schemes, which are preferred schemes to theinvention, an image is partitioned into rectangular blocks of e.g. 8×8pixels. Each block is transformed separately with a two-dimensional DCT.Resulting DCT coefficients are quantized and transmitted or stored in aprogressive manner, so that the most important information istransmitted first. This can be done by successive quantization, wherethe coding residue is reduced step by step. After the transformation,most of the energy of the image is concentrated in the low-frequencycoefficients, and the rest of the coefficients have very small values.This means that there are many zeros in most significant bit planes.

A bit-plane (BP) is a plane that comprises bits of the transmissioncoefficients with certain significance. An example of such a bit-planeis shown in FIG. 1. This bit-plane BP comprises a bit for each transformcoefficient (8×8) with certain significance. A bit-plane comprising mostsignificant bits of all transform coefficients is called a mostsignificant bit plane (BP_(MSB)). Further, less significant bits formrespective less significant bit-planes. FIG. 1 further shows arectangular scan zone with dimensions: R_(MAX)=3 and C_(MAX)=4, whereinR_(MAX) is a maximum row number and C_(MAX) is a maximum column number.Note that position (0,0) represents a bit of a DC coefficient.

A graphical visualization of a coding method according to the inventionis presented in FIG. 2, which method will be explained below. In thisembodiment the DC coefficient is transmitted entirely at the beginningof the bit-stream. The other DCT coefficients are encoded andtransmitted bit-plane by bit-plane, starting with the most significantbit-plane BP_(MSB) (not counting the sign plane). Although this is apreferred embodiment for e.g. hybrid encoders, it is also possible toencode the DC coefficient in the same way as the other DCT coefficients,i.e. bit-plane by bit-plane. While encoding each bit-plane, adistinction is made between significant and insignificant coefficients.A significant coefficient (SC) is a coefficient for which one or morebits have already been transmitted (in a more significant bit plane). Aninsignificant coefficient is a coefficient for which no bits have beentransmitted yet. That is the case if all bits in previous bit-planeswere zeros. As long as a coefficient has zeros, it is regarded asinsignificant.

The significant and insignificant coefficients have differentprobability distributions. A bit in the current bit plane of asignificant coefficient has about equal probability of being a zero or aone. Therefore, there is not much to be gained by trying to moreefficiently transmit it. A bit in an insignificant coefficient, however,is very likely to be a zero. Furthermore, because of the properties ofthe DCT (and typical images), the significant and insignificantcoefficients tend to be clustered. This enables us to efficientlytransmit many “insignificant zeroes” by a zoning technique.

Initially all coefficients are marked insignificant. Then, starting withthe most significant bit plane BP_(MSB), the encoding is started. Anindication (e.g. one bit as shown in FIG. 2: 0 or 1) is transmittedwhether any insignificant bits are found in the current bit-plane thatbecome significant, i.e. when a previously insignificant coefficient hasa non-zero bit. If these so-called newly significant coefficients (NSC)have been found, their positions are transmitted with aid of therectangular scan zone as shown in FIG. 1. After the positions of thenewly significant coefficients, their sign bits are transmitted. A wayof transmitting the positions of the NSC is given below. The bits forthe newly significant coefficients do not have to be transmitted,because they are always one. Otherwise, the coefficient would haveremained insignificant. The above-described procedure is repeated foreach bit plane (BP_(MSB) . . . BP_(LSB)) until a certain stop criterionhas been met, e.g. a certain bit-rate or quality or just because allbit-planes (BP_(MSB) . . . BP_(LSB)) have been put into the bit-stream.

For a certain bit-plane, bits of significant coefficients (zeros andnon-zeros) are transmitted automatically before the indication is sendwhether or not newly-significant coefficients are present in the currentbit-plane. Because all coefficients are marked insignificant at thestart of the procedure, for the most significant bit-plane BP_(MSB) nosignificant coefficients exist and only bits are transmitted for thenewly significant coefficients. These newly significant coefficients arethen marked significant. This means that when the next bit-plane isprocessed these coefficients are significant and their bits aretransmitted automatically. If no newly significant coefficients arefound, an indication is sent (e.g. zero-bit) and the coding proceedswith the next bit-plane.

As mentioned above, to encode the positions of newly significantcoefficients, the rectangular scan zone is used. The scan zone indicatesthe area in which newly significant coefficients have been found. Thedimensions of the scan zone are determined by outermost positions of thenewly significant coefficients. With reference to FIG. 1, R_(MAX)indicates the maximum row number (here: 3) and C_(MAX) the maximumcolumn number (here: 4) wherein a newly significant coefficient has beenfound. Because the scan zone only indicates the maximum area in whichnewly significant coefficients have been found, exact positions stillneed to be transmitted. This is done by sending a single bit for eachnewly significant coefficient inside the scan zone to indicate whetheror not the coefficient has become significant. Since it is not necessaryto transmit a bit for the coefficients that were already significant(nor for the DC coefficient in case this coefficient has beentransmitted separately), this position coding is very efficient.

The encoding is based on the observation that the coefficients withlarger magnitude tend to be close with the lower horizontal or verticalfrequencies. Therefore, in the prior art a zig-zag or a diagonal scanorder is used. These scan orders are signal-independent and assume thatthe data is concentrated in the upper left triangular zone of thetransformed block. Although this assumption is true on average, theinvention is based on the insight that individual DCT blocks often havea bias for either the horizontal or the vertical direction. Therefore, asignal dependent rectangular scan zone as described above (alsooriginating in the upper left corner, i.e. lower frequencies) produces amore efficient encoding of the coefficients. The scanning of this zonecan be done in any fashion as long as an encoder and a correspondingdecoder are synchronized on this scanning. Possible scan orders are,e.g., diagonal, zigzag, vertical or horizontal.

The rectangular scan zone can differ from one block to another block,but also from one bit-plane to another bit-plane within the same block.When no newly-significant coefficients are found within a block for acertain bit-plane, no scan zone is defined. In this case, only a bit istransmitted to indicate that no newly significant coefficients arepresent.

Note that in the above-described embodiment, the significantcoefficients of a current bit-plane are inserted in the bit-streambefore the zoning information and newly significant coefficients for thesame bit-plane. This order may be changed without departing from thescope of the invention. The significant coefficients can for example beinserted in the bit-stream after the information concerning the newlysignificant coefficients.

An increase in coding performance can be achieved at the cost ofadditional complexity if a part of the bit-stream representing the newlysignificant coefficients is entropy coded, e.g. arithmetic coded. Thezoning information can for example be Huffman coded.

FIG. 3 shows an exemplary embodiment according to the inventioncomprising a DCT transformer 1, a scalable coder 2 and a truncator T 3.The scalable coder 2 comprises: a bit plane switch detector (BPS) 20, ascanning unit (R_(MAX)/C_(MAX)) 21, an index of significant coefficients(ISC) 22 and an output multiplexer 23. An input signal, describing adigitized image, is DCT transformed in the DCT 1 resulting in a signalS. In the scalable coder 2 a DC coefficient is furnished to the outputmultiplexer 23. After the DC coefficient has been furnished to themultiplexer 23, the bits of the significant coefficients SC indexed inthe ISC 22 are forwarded for the current bit-plane detected in BPS 20.In the scanning unit 21, the rectangular scan zone is selected for anynewly significant coefficients from the most significant bit-planeBP_(MSB) to the least significant bit-plane BP_(LSB). If newlysignificant coefficients are present, the values for R_(MAX) and C_(MAX)are determined and furnished to the multiplexer 23, followed by bits forthe newly significant coefficients NSC. These bits comprise the positionand sign bits of the NSC as already discussed. This process is repeatedfor each next bit-plane detected in the BPS 20. The NSC are indexed inISC 22 and regarded as significant in the next bit-plane. The producedscalable bit-stream O can than be truncated to a desired bit-rate bysimply truncating the bit-stream at a desired position in truncator 3.

An entire image composed of (DCT) blocks can be coded by coding all DCTblocks separately and concatenating them in a scanning fashion. Anotherway of coding an entire image is shown in FIG. 4. The image istransmitted in a scalable way, not by concatenating (and truncating ifdesired) the separate DCT blocks (DCT_1 to DCT_N), but by cyclicallyscanning the coded DCT blocks and transmitting only a part of the codedtransform coefficients (P1, P2, . . . ), e.g. one or a few bits, of theindividual blocks DCT_i with i=1 to N. A next scanning pass then obtainsa next part of the coded transform coefficients of the DCT blocks. Thenumber of bits in the selected parts can differ for each block or eachscanning pass, e.g. depending on the significance of the part of thecoded transform coefficients, as illustrated in FIG. 4 for part P3. Itis possible to select some bits that represent data of certainsignificance or a certain coefficient, which are represented by adifferent amount of bits for different blocks. If a certain DCT blockdoes not have a coded part of certain significance required in thescanning pass, the specific DCT block may be skipped. This isillustrated in FIG. 4 where the block DCT_2 is skipped in the thirdscanning pass (P3) because DCT_2 does not contain any coded transformcoefficients anymore, i.e. the code is exhausted. It is also possible toskip a block in a certain scanning pass, because the significance isonly lower than required, illustrated in the fourth scanning pass forDCT_4. It still remains possible to select a next part of the codedtransform coefficients of this block DCT_4 in a next scanning pass. Inthis way, a scalable coding of an entire image is obtained instead ofthe block-wise scalable coding as obtained when all DCT blocks areindividually coded and concatenating in a scanning fashion.

Hybrid video compression schemes, such as MPEG2 and H.263 use an imagememory for motion-compensated coding. In VLSI implementations, thisimage is usually stored in external RAM because of its large size. Toreduce overall system costs, a compression of the image is proposed by afactor 4 to 5 before storage, which enables embedding of the imagememory on the encoder IC itself. In a DCT domain encoder, the inputsignal is directly subjected to a DCT outside of the encoding loop. (seeFIGS. 5 and 6). This means that motion estimation and compensation needto be performed in the DCT-domain. The local decoding only goes as faras performing a de-quantization (IQ) and inverse MC (IMC). To takeadvantage of the large number of zero coefficients after quantization(Q) (still present after IQ), a scalable coder (LLC) according to theinvention (similar to the scalable coder 2 shown in FIG. 3) is usedbefore storage. A scalable coding method is inherently lossless, but canbe quantized from the bit-stream if necessary. Extraction from a memory(MEM) for motion-compensation is performed by a scalable decoder (LLD).Note that almost all of the encoder parts are now situated in theDCT-domain whereas for a traditional, non-DCT domain encoder only alimited part is situated in the DCT-domain.

To control and guarantee the actual storage, scalable compression isused as described above.

FIG. 5 shows a camera system comprising a first DCT domain hybridencoder according to the invention. The hybrid encoder is in this case aso-called ‘PIPI’ encoder indicating that it encodes alternating I(intra) and P (inter) frames. The camera system comprises a camera 4 anda hybrid encoder 5. A signal generated by the camera 4 is first DCTtransformed in DCT 50. Thereafter, the transformed signal is subjectedto motion estimation in ME 51 and to motion compensation in MC 52. Themotion compensated signal is quantized in Q 53. The quantized signal isfurther processed by a zig-zag scanner (ZZ) 58, a run-length encoder(RLE) 59 and a variable length encoder (VLE) 60 to obtain, e.g., an MPEGencoded signal. The quantized signal is further scalable coded in an LLC54 and thereafter furnished to a memory 55. The required size of thememory 55 can be guaranteed by the buffer/rate control mechanism of theencoder 5 itself. This is because in effect only coefficients of anintra frame I are stored in the memory 55. For applications whereencoder cost and edit-ability are more important than compression ratio,such as storage applications, this is a suitable encoder. The loopmemory 55 is placed just after the quantizer 53 (via the LLC 54), takingalmost full advantage of the parent encoder efforts. To obtain areconstructed frame that can be used in the motion estimator 51, theencoder further comprises a scalable decoder LLD 56 and an inversequantizer IQ 57, both coupled to the memory 55. The scalable decoder LLD56 performs an inverse operation of the scalable coder LLC 54.

At higher compression ratios, required for lower bit-rates, successive Pframes must be used. An architecture of a camera system comprising amultiple P frame encoder 7 is shown in FIG. 6. Similar to FIG. 5, theencoder 7 comprises a DCT 70, an ME 71, an MC 72, a Q 73, a ZZ 80, a RLE81 and a VLE 82. The Q 73 is coupled via an IQ 74 to an inverse motioncompensator (IMC) 75 to obtain a reconstructed signal. In betweeninter-coded frames P an undefined number of non-zero coefficients cannow slip through the IMC mechanism 75 directly to a loop memory 78,bypassing the Q 73. A method to actively control the storage demands isto quantize the signals going into the loop memory 78. Some amount ofquantization is permissible as long as the image quality stays(significantly) higher than the targeted output quality of the encoder,and the number of successive P frames is limited. This quantizing isperformed by simply stripping a certain percentage of the bit-stream foreach DCT block, according to the scalable coding principle. A separatebuffer control mechanism can profile the image contents and adjust thispercentage on the fly. The quantization information is not needed forthe decoding phase that is performed in a LLD 79. The additionalquantizing is performed by truncator T 77 on a scalable bit-streamproduces by an LLC 76. A fall-back mechanism may be employed byswitching to intra-blocks if the number of non-zero coefficients ishigher than can be accepted. The embodiments shown in FIGS. 5 and 6produce a standard MPEG or similar encoded bit-stream. This bit-streamcan be decoded by a standard decoder.

Although in the aforementioned embodiments scalable coding is usedwithin an encoder, i.e. to furnish a scalable bit-stream to aloop-memory, scalable coding can also be used for transmitting ascalable bit-stream to a remote decoder. The receiver then needs meansfor decoding the scalable bit-stream. FIG. 7 shows a camera systemcomprising the camera 4 and a hybrid encoder 9. The hybrid encoder 9comprises: a motion estimator (ME) 90, a motion compensator (MC) 91, aDCT transformer 92, a scalable coder (LLC) 93 (see FIG. 3), an entropycoder (EC) 94 (optional) and a truncator (T) 95. The encoder furthercomprises an entropy decoder (ED) 96 (optional), a scalable decoder(LLD) 97, an inverse DCT transformer (IDCT) 98, an inverse motioncompensator (IMC) 99 and a memory (MEM) 100. Instead of the standardzig-zag scanning, run-length coding and variable length coding, thescalable coder LLC 93 is used to furnish a scalable bit-stream to anoutput of the hybrid coder 9. The scalable bit-stream is entropy codedin EC 94, e.g. arithmetic or Huffman coding. The embodiment according toFIG. 7 comprises a truncator (T) 95 in the output path that truncatesthe scalable bit-stream to obtain an output bit-stream BS with a desiredbit-rate. This embodiment provides a convenient, low-complexity bit-ratecontrol that adapts the bit-rate faster and better than an embodimentthat uses a feedback loop to adapt a quantizer. Combinations of theembodiment shown in FIG. 7 with the embodiments shown in FIGS. 5 and 6are feasible. If motion compensation is not required, an embodimentmainly comprising the DCT 92, the scalable coder LLC 93 and thetruncator T 95 may be used (similar to FIG. 3).

Because the output of the embodiment of FIG. 7 is not a standard MPEG-2output, a non MPEG-2 standard decoder is required to decode thebit-stream BS. A receiver comprising a scalable decoder 11 is shown inFIG. 8. A scalable bit-stream BS is received in the decoder 11, inparticular in the entropy decoder ED 111. The source of the bit-streamBS may be a storage medium 10, but can also be a transmission over somekind of medium. After entropy decoding, the bit-stream BS is scalabledecoded in LLD 112. Further elements of the decoder are, an in verse DCTtransformer (IDCT) 113, an inverse motion compensator (IMC) 114 and amemory (MEM) 115 which are similar to their counterparts in the encoder9. Bits of coefficients that are affected by a truncation can be set tozero, to expected values or to random values by the decoder 11. Thedecoded bit-stream can be displayed on a display D 12. Depending on thecomplexity of the encoder, the ED 111 or the IMC 114 and the MEM 115 canbe omitted.

In summary, the invention proposes a method of and a device for coding asignal to obtain a scalable bit-stream. The signal comprises blocks ofvalues. Each block is represented as a sequence of bit planes and thevalues are scanned and transmitted in an order of decreasing bit planesignificance. For each bit plane the scanning and transmitting areperformed in a rectangular scan zone starting from a corner of the block(usually an upper-left corner). Preferably, the scanning andtransmitting is performed on each block individually. The producedbit-stream is quantized to a desired bit-rate by simple truncating thebit-stream at a desired position. Initially all values are markedinsignificant. For each bit-plane, a bit is transmitted for eachsignificant value, i.e. a value that has been newly significant in aprevious bit-plane. Besides the significant values, an indication istransmitted whether or not any insignificant values become newlysignificant in the current bit plane. The dimensions of the rectangularscan zone are selected and transmitted for the newly significant valuesin the current bit plane. This is followed by an indication for each notpreviously significant value inside the scan zone whether the value hasbecome newly significant and a sign bit for each newly significantvalue. After that a next bit plane is processed. The values may betransform coefficients.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. Images may be divided in sub-images,wherein the invention is applied to the sub-images rather than theimage. In the claims, any reference sings placed between parenthesesshall not be construed as limiting the claim. The word “comprising” doesnot exclude the presence of other elements or steps than those listed ina claim. The invention can be implemented by means of hardwarecomprising several distinct elements, and by means of a suitablyprogrammed computer. In a device claim enumerating several means,several of these means can be embodied by one and the same item ofhardware.

1. A method of coding (2) a signal (S) comprising blocks of values toobtain a scalable bit-stream (O,BS), the method comprising the steps of:representing (20) each block as a sequence of bit planes (BP), whereinmost significant bits of the values form a most significant bit plane(BP_(MSB)) and respective less significant bits of the values formrespective less significant bit planes; and scanning and transmitting(21,23) significant coefficients values in an order of decreasing bitplane (BP) significance; wherein for each bit plane the step of scanningand transmitting (21,23) is performed in a rectangular scan zone(R_(MAX), C_(MAX)) starting from a corner of the block, wherein R_(MAX)represents a maximum row number and C_(MAX) represents a maximum columnnumber determined as the outermost positions of newly significantcoefficients within each bit plane and said R_(MAX) and C_(MAX) valuesare transmitted in said bit-stream.
 2. The method as claimed in claim 1,wherein the values are transform coefficients.
 3. The method as claimedin claim 1, wherein the coding (2) is performed on each blockindividually to obtain respective scalable bit-streams for respectiveindividual blocks.
 4. The method as claimed in claim 1, wherein the stepof scanning and transmitting (21,23) comprises: initially marking (22)all values insignificant; and performing the following steps for eachbit-plane (BP_(MSB) . . . BP_(LSB)) until a stop criterion is met:transmitting (22,23) a bit for each significant value (SC) in a currentbit plane (BP); transmitting (21,23) an indication whether or not anyinsignificant values become newly significant in the current bit plane;and selecting and transmitting (21,23) an indication for each notpreviously significant value inside the scan zone whether the value hasbecome newly significant (NSC) and a sign bit for each newly significantvalue (NSC) following said transmitted R_(MAX) and C_(MAX).
 5. Themethod as claimed in claim 4, wherein parts of the bit-streamrepresenting the newly significant values (NSC) are entropy coded. 6.The method as claimed in claim 3, wherein a scalable bit-stream isobtained by cyclically and sequentially scanning selected parts (P1,P2,. . . ) of the respective scalable bit-streams DCT_1 . . . DCT_N) of therespective individual blocks.
 7. A device (2) for coding (2) a signal(S) comprising blocks of values to obtain a scalable bit-stream (O,BS),the device comprising: means for representing (20) each block as asequence of bit planes (BP), wherein most significant bits of the valuesform a most significant bit plane (BP_(MSB)) and respective lesssignificant bits of the values form respective less significant bitplanes; and means for scanning and transmitting (21,23) the values in anorder of decreasing bit plane (BP) significance; wherein for each bitplane the means for scanning and transmitting (21,23) have been arrangedto perform the scanning and transmitting for each bit plane in arectangular scan zone (R_(MAX), C_(MAX)) starting from an upper leftcorner of the block wherein R_(MAX) represents a maximum row number andC_(MAX) represents a maximum column number determined as the outermostpositions of newly significant coefficients within each bit plane andsaid R_(MAX) and C_(MAX) values are transmitted in said bit-stream. 8.The device (93) as claimed in claim 7, further comprising: a truncator(95) for truncating the scalable bit-stream (O,BS) at a certainbit-rate.
 9. The device (54,76) as claimed in claim 7, furthercomprising: a memory (55,78) for storing a previous frame-wherein thescalable bit-stream (O,BS) is furnished to the memory (55,78).
 10. Acamera system comprising: a camera (4); and an encoder for coding (2) asignal (S) comprising blocks of values to obtain a scalable bit-stream(O,BS), the device comprising: means for representing (20) each block asa sequence of bit planes (BP), wherein most significant bits of thevalues form a most significant bit plane (BP_(MSB)) and respective lesssignificant bits of the values form respective less significant bitplanes; and means for scanning and transmitting (21,23) the values in anorder of decreasing bit plane (BP) significance; wherein for each bitplane the means for scanning and transmitting (21,23) have been arrangedto perform the scanning and transmitting for each bit plane in arectangular scan zone (R_(MAX), C_(MAX)) starting from an upper leftcorner of the block, wherein R_(MAX) represents a maximum row number andC_(MAX) represents a maximum column number determined as the outermostpositions of newly significant coefficients within each bit plane andsaid R_(MAX) and C_(MAX) values are transmitted in bit-stream.
 11. Amethod of decoding (11) comprising: receiving (111) a scalablebit-stream (O,BS) comprising blocks of values, the values for each blockbeing available in an order of decreasing bit plane significance and foreach bit plane scanned in a rectangular scan zone (R_(MAX), C_(MAX))starting from an upper left corner of the block, wherein R_(MAX)represents a maximum row number and C_(MAX) represents a maximum columnnumber determined as the outermost positions and received in thebit-stream; regenerating (112) the blocks of values from the scalablebit-stream (O,BS); and decoding (113–115) the blocks of values.
 12. Ascalable decoder (11) comprising: means for receiving (111) a scalablebit-stream (O,BS) comprising blocks of values, the values for each blockbeing available in an order of decreasing bit plane significance and foreach bit plane scanned in a rectangular scan zone (R_(MAX), C_(MAX))starting from an upper left corner of the block, wherein R_(MAX)represents a maximum row number and C_(MAX) represents a maximum columnnumber determined as the outermost positions of newly significantcoefficients within each bit plane and received in the bit-stream; meansfor regenerating (112) the blocks of values from the scalable bit-stream(O,BS); and means for decoding (113–115) the blocks of values.
 13. Thedecoder as claimed in claim 12 further comprising: means for outputting(12) the decoded values.
 14. A method for scanning a scalable bit-stream(BS) comprising blocks of values, the values for each block beingavailable in an order of decreasing bit plane significance, said methodcomprising the step of: scanning each bit plane in a rectangular scanzone (R_(MAX), C_(MAX)) starting from an upper left corner of a selectedblock, wherein R_(MAX) represents a maximum row number and C_(MAX)represents a maximum column number determined as the outermost positionsof newly significant coefficients within each bit plane and said R_(MAX)and C_(MAX) values are transmitted in said bit-stream.
 15. The method asrecited in claim 14, wherein said bit-stream (BS) is recorded on astorage medium (10).